Method and apparatus for driving a brushless DC motor

ABSTRACT

A drive circuit for a brushless DC motor includes a switch constructed and arranged to drive the motor with a pulse signal responsive to a control signal, and control circuitry coupled to the switch and constructed and arranged to generate the control signal responsive to rotor position information from the motor so as to synchronize the pulse signal to the rotor position. A current sensing device can be used to provide the rotor position information to the control circuitry by sensing current flowing through the motor.

[0001] This application claims priority from U.S. ProvisionalApplication No. 60/290,397 filed May 10, 2001, which is incorporated byreference.

BACKGROUND

[0002] Brushless direct current (DC) motors typically include electroniccircuitry that energizes and de-energizes electric coils (windings) inthe motor in order to make the rotor spin. Brushless DC motors arecommonly used to drive cooling fans in electronic devices such aspersonal computers (PCs). A typical brushless DC motor used in a PC ispackaged in such a way that only two terminals are accessible: apositive power supply terminal VS and a ground terminal GND (alsoreferred to as a positive and a negative rail, respectively). A thirdterminal which provides a signal that indicates the speed of the motoris sometimes accessible as well.

[0003] Cooling fans driven by brushless DC motors have traditionallybeen run at full speed at all times since this is the simplestimplementation. In a typical PC, this is accomplished by simplyconnecting the GND terminal to a power supply ground, and the VSterminal to the computer's +12 Volt or +5 Volt power supply. This is aninefficient scheme, however, since most electronic devices only requiremaximum cooling power for short periods of time and at random intervals.Running the fan constantly at fall speed wastes energy and generatesunnecessary noise.

[0004] A recent trend is to run the fan motor at different speedsdepending on the cooling demand. One way to accomplish this is to drivethe motor with a variable voltage power supply. This is sometimesreferred to as linear fan speed control. Linear fan speed control,however, can be difficult and expensive to implement because it requiresa variable voltage power supply. There are also problems such as thoseassociated with fact that most 12 Volt fans must initially be drivenwith at least 6 to 8 Volts to overcome the initial resistance torotation.

[0005] Another solution involves the use of pulse width modulation(PWM). In a PWM scheme, the power supply to the motor is repetitivelyturned on and off at a fixed frequency but variable duty cycle. When thepower supply signal has a relatively low duty cycle, for example 25percent (that is, the power supply is on 25 percent of the time and off75 percent of the time), the motor to turns at a relatively slow speed.Increasing the duty cycle causes the motor to spin faster. Full power isachieved by leaving the power supply signal on at all times, i.e., 100percent duty cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a simplified pictorial drawing of a prior art brushlessDC motor.

[0007]FIG. 2 is a timing diagram illustrating a prior art technique fordriving a brushless DC motor at full speed.

[0008]FIG. 3 is a timing diagram illustrating a prior art technique fordriving a brushless DC motor at a reduced voltage and speed.

[0009]FIG. 4 is a timing diagram illustrating a prior art PWM techniquefor driving a brushless DC motor at a reduced speed.

[0010]FIGS. 5 and 6 are timing diagrams illustrating an embodiment of amethod in accordance with the present invention for driving a brushlessDC motor.

[0011]FIG. 7 is a block diagram of an embodiment of a brushless DC motordrive circuit in accordance with the present invention.

[0012]FIG. 8 is a block diagram of another embodiment of a brushless DCmotor drive a circuit in accordance with the present invention.

[0013]FIG. 9 is a flow diagram illustrating an embodiment of a method inaccordance with the present invention for starting a brushless DC motor.

[0014]FIG. 10 is a timing diagram illustrating another embodiment of amethod in accordance with the present invention for driving a brushlessDC motor.

[0015]FIG. 11 is a timing diagram illustrating yet another embodiment ofa method in accordance with the present invention for driving abrushless DC motor.

[0016]FIG. 12 is a timing diagram illustrating a further embodiment of amethod in accordance with the present invention for driving a brushlessDC motor.

[0017]FIGS. 13 and 14 illustrate the time between tachometer pulses fora three-phase brushless DC motor and a two-phase brushless DC motor,respectively.

[0018]FIG. 15 is a flow diagram illustrating an embodiment of a methodin accordance with the present invention for driving a brushless DCmotor.

[0019] FIGS. 16-19 illustrate the time every pulse, every other pulse,every third pulse, and every fourth pulse, respectively, from atachometer in a brushless DC motor.

DETAILED DESCRIPTION

[0020] Although prior art PWM schemes are relatively easy andinexpensive to implement, they generates vibrations that can damage themotor and other components in the system. They also generate audiblenoises such as ticking noises. These problems are believed to be causedby stresses that occur when the pulse width modulated power supplysignal is switched at inopportune times.

[0021] These stresses can be understood by first considering theoperation of a typical brushless DC motor such as the two-phase motorshown in FIG. 1. One set of windings in the stator 50 is designated a-a′(also referred to as phase a), and another set is designated b-b′ (alsoreferred to as phase b). Internal electronic circuitry in the motoroperates on the assumption that the power supply signal is a constant DCvoltage. The circuitry alternately energizes phase a and phase b,depending on the position of the rotor 52, which rides on bearing 54.When one phase is energized, it generates a magnetic field that attractsone pole of the rotor, thereby creating a torque that causes the rotorto spin. When the rotor reaches a certain position, the internalcircuitry switches the first phase off and energizes the other phase soas to generate a magnetic field that attracts the other pole of therotor. To minimize stresses in the motor, the phases are switched whenthe rotor reaches a minimum torque position. The internal circuitrygenerally senses the rotor position by using a position sensing devicesuch as a hall-effect sensor that generates a position signal TACH asshown in FIG. 2.

[0022] When the speed of a brushless DC motor is reduced by lowering thepower supply voltage, the magnetic field created by each phase isweaker, so the rotor spins at a lower speed. The internal circuitry hasno problem switching the phases at the proper times because it canalways detect the rotor position using the TACH signal as shown in FIG.3. As discussed above, however, it is expensive and difficult to providea variable voltage power supply in many electronic systems.

[0023] In a prior art PWM scheme, a fixed frequency (30 Hz for example)is selected for the PWM signal. FIG. 4 illustrates a PWM power supplysignal having a 25 percent duty cycle which typically causes the motorto turn at about half speed. When the PWM signal of FIG. 4 is applied toa brushless DC motor, the internal circuitry energizes the phases asshown by the trace labeled TACH. When the TACH trace shown in FIG. 4 isat “a on” or “b on”, phase a or phase b is energized, respectively. Whenthe trace is at the midpoint “off”, neither phase is energized becausethe PWM power supply signal is off during that time. The tick marks atthe bottom of FIG. 4 indicate the minimum torque points at which thephases should optimally be switched.

[0024] Since the PWM power supply signal is free running, that is, notsynchronized to anything, the phases are energized at positions ofrandom torque, and sometime maximum torque, between the rotor and thestator. This can cause several problems. First, bearings in the motorrely on a nominal point contact between a race and a ball. The bearingsare easily damaged by high instantaneous torque which causes impactloading between the ball and race. This creates indentations known asBrinell marks in the race. Brinell marks quickly become potential sitesfor structural damage, thereby reducing the overall reliability of themotor.

[0025] Second, energizing a phase at a high torque position produces atorque burst that causes minute flexing of the entire motor structure,thereby resulting in an audible ticking noise. The amount of noisedepends on the motor speed, the frequency of the PWM power supplysignal, and the duty cycle, all of which change depending on theparticular configuration.

[0026] Third, if the fixed frequency of the PWM power supply signalhappens to be a harmonic of the rotational speed of the motor, thewindings are energized at the same place during each revolution. Thiscan cause the motor to shake, thereby causing further damage to both themotor and other apparatus to which the motor is attached.

[0027] One aspect of the present invention involves synchronizing pulsesin a power supply signal with the position of the rotor in a brushlessDC motor. FIGS. 5 and 6 illustrate the operation of an embodiment of amethod for synchronizing pulses with rotor position in accordance withthe present invention. Referring to FIG. 5, the power supply signal VSis a pulse width modulated signal having a train of pulses with a 25percent duty cycle. Rather than generating the pulses on a free-runningbasis, however, they are synchronized to the position of the rotor. Asshown in FIG. 5, each pulse begins at a minimum torque position. Thiscauses the internal circuitry in the brushless DC motor to energize thewindings at times that minimize stress in the motor.

[0028] When the motor is energized, instantaneous torque ischaracterized as follows:$T = {{- \frac{P}{2}}L_{sr}i_{s}i_{r}{\sin \left( {\frac{P}{2}\theta_{m}} \right)}}$

[0029] where:

[0030] T=Torque in Newton-meters (a negative sign means that the electromagnetic torque acts in the direction that brings the magnetic fields ofthe stator and rotor into alignment);

[0031] P=number of poles;

[0032] L_(sr)=the mutual inductance when the magnetic axes of the statorand rotor are aligned;

[0033] i_(s)=current in the stator;

[0034] i_(r)=current in the rotor;

[0035] θ_(m)=actual mechanical angle between the rotor and stator.

[0036] For a given permanent magnet AC motor, P, L_(sr), i_(s) and i_(r)are constant. This reduces the equation to T=K*sin(θ_(m)). If the phaseschange when the torque is zero, it will not cause any undo torque in thesystem. Because K is a constant, the only thing that can be controlledis θ_(m). The angle θ_(m) can be controlled when the motor is energized.In order to set the equation to a minimum, θ_(m) must be equal to zero.Since the tachometer signal is also a relative position signal, the fancan be energized with a pulse stream that is synchronous with thetachometer.

[0037] As the duty cycle of the power supply signal increases, therotational speed of the motor increases. Therefore, the frequency of thepulses in the power supply signal is increased accordingly so that thepulses remain synchronized with the rotor position as shown in FIG. 6.The duty cycle of the VS signal in FIG. 6 is about 55 percent(corresponding to about 75 percent speed).

[0038]FIG. 7 is a block diagram of an embodiment of a drive circuit fora brushless DC motor in accordance with the present invention. The drivecircuit 10 receives input power 12 from any suitable source, typically afixed voltage power supply. The drive circuit generates a power supplysignal 14 having a series of pulses for driving a brushless DC motor 16.The drive circuit receives rotor position information 18 from the motorto enable the drive circuit to synchronize the pulses with the rotorposition.

[0039] Different techniques can be used to determine the rotor position.If the motor has a position signal that is accessible (from a digitaltachometer for example), the drive circuit can read the rotor positionby directly monitoring the position signal.

[0040] A technique for determining the rotor position in accordance withthe present invention is illustrated in FIG. 8. The drive circuit 16shown in FIG. 8 includes a switch 20 (shown here as a field effecttransistor) arranged to turn power to the motor on and off in responseto a PWM control signal PWMCTRL from control circuitry 24. A currentsensing device 22 (shown here as a current sensing resistor) is arrangedin series with the switch to provide a current feedback signal IFB tothe control circuitry. Alternatively, the parasitic on resistance of theswitch could be used to sense current. By monitoring the current flowingthrough the motor, the position of the rotor can be determined. Minimumtorque positions occur when the mechanical angle between the rotor andstator (θ_(m)) is zero. At this instant, a commutation current pulse isdetected. This technique eliminates the need for a separate positionsignal from the motor.

Start-up Sequence

[0041] In prior art PWM control schemes for brushless DC motors, thepower supply signal is usually driven at full power (i.e., not pulsed)for a fixed period of time at start-up, typically in the range of a fewmilliseconds to a few seconds, to allow the motor to come up to fullspeed. The power supply signal is then pulse width modulated to operatethe motor at the required speed. Since different motors have differentstart up times, the fixed period of start-up time for prior art PWMmotor drives is typically made longer than necessary to assure that itwill be long enough for the slowest starting motors. This is inefficientand generates unnecessary noise.

[0042]FIG. 9 illustrates an embodiment of a start-up sequence for a PWMcontrol scheme in accordance with the present invention. First, themotor is turned on at full power, i.e., the power supply signal isconstantly on (not pulsed), at 100. The number of motor poles isdetermined at 102. This determination can be skipped if the number ofpoles is already known. At 104, the speed of the motor is monitoreduntil it reaches a suitable speed. The motor is then driven with a PWMpower supply signal at 106.

[0043] One method for determining when the motor has reached a suitablespeed in accordance with the present invention is to count the number oftachometer edges from a tachometer signal. Since a given motor typicallytakes a certain number of rotations to come up to speed, this provides arough approximation of the motor speed.

[0044] A more sophisticated technique for determining when the motor hasreached a suitable speed in accordance with the present invention is tomeasure the time between tachometer edges. Since the number of poles isknown, the motor speed can be accurately calculated based on the timebetween tachometer edges. An advantage of this method is that itoptimizes the start-up time. That is, the power supply signal isswitched from constant-on to PWM operation just as soon as the motorreaches a suitable speed.

[0045] As used herein, tachometer edge or pulse refers not only an edgeor pulse in a position signal from an actual tachometer, but also moregenerally to anything that signifies events relating to the position ofthe rotor. Thus, if the current monitoring scheme described above withreference to FIG. 8 is utilized instead of a Hall-effect tachometer,instants of minimum torque would essentially be considered tachometeredges.

Steady-state Operation

[0046]FIG. 10 illustrates another embodiment of a method for driving abrushless DC motor in accordance with the present invention once themotor has started. The top trace in FIG. 10 indicates the physicalrotation of the motor where θ1 indicates the amount of time the motortakes for a first rotation, θ2 is for the second rotation, etc. Thesecond trace indicates the undisturbed tachometer signal which providesposition and velocity information. The third trace illustrates the PWMpower supply signal driving the motor. A and C indicate on times,whereas B and D indicate off times. The example shown in FIG. 10 is fora six-pole (three phase) motor (i.e., six “on” times per revolution).The bottom trace illustrates the actual tachometer output signal fromthe motor, taking into account the fact that the power supply signal tothe motor is being switched on and off to control the speed. The actualtachometer output signal is used to determine the amount of time ittakes the motor to complete one rotation.

[0047] The normal on time A₁ and normal off time B₁ for the firstrotation are calculated as follows:

θ1/P=A ₁ +B ₁

[0048] where P is the number of poles in the motor. The duty cycledetermines the relationship between A and B:

A ₁ =DC(A ₁ +B ₁)

B ₁=(1−DC)(A ₁ +B ₁)

[0049] where DC is the duty cycle (percentage on time).

[0050] During the second rotation (θ2), the PWM power supply signal isturned on during times A1 and off during times B1. At the end of thelast on time A1, the power supply signal is turned off for a shortened“off” time D1, and then turned on for an indeterminate amount of timeuntil a tachometer edge is detected, and then for an additional amountof time equal to A1. As a result, on time C1 is longer than A1. Byturning the power supply signal on slightly earlier than needed duringthe last tachometer cycle, it assures that power to the motor will beswitched on before the tachometer edge marking the end of the completerotation. This assures that the entire PWM power supply signal can beresynchronized at the end of each rotation. The “D” off times should beshorter than the “B” off times by as little as possible while stillallowing an adequate margin to accommodate changing rotational speeds.Using D=0.75B has been found to provide reliable results. Theresynchronization can be accomplished with suitable position sensingtechnique such as the current monitoring scheme described above.

[0051] The motor speed is controlled by varying the duty cycle DC. Aftera complete revolution is completed, the duty cycle is updated, and theon and off times for the next revolution are recalculated.

[0052] The methods described herein can be used with brushless DC motorshaving any number of poles, and not all poles need be utilized. That is,the motor can be driven by using fewer than all of the poles. Forexample, in the technique described above with respect to FIGS. 5 and 6,the motor can be driven using only phase a and leaving phase b off asshown in FIG. 11. This can be helpful in applications where highresolution is required at the low end of the operating range.

[0053] Another method in accordance with the present invention involvesthe use of multiple pulses on each phase during a single revolution. Anexample embodiment of such a technique is illustrated in FIG. 12.

Determining Number of Poles

[0054] Further aspects of the present invention involve determining thenumber of poles in a brushless DC motor. The poles in a brushless DCmotor are arranged almost symmetrically around the stator. However, thepoles are not spaced at exactly even intervals to assure that the motorwill begin rotating at start-up regardless of the position in which therotor stopped previously. This asymmetry causes slight variations in thetime between tachometer edges. By measuring the time between tachometeredges and looking for patterns, it is possible to determine the numberof poles in the motor.

[0055]FIG. 13 illustrates the times between successive tachometer edgesfor a six pole (three-phase) brushless DC motor running at a steadyspeed. A three-phase motor provides three tachometer pulses perrevolution. The vertical axis is the time between successive pulses inmicroseconds, and the horizontal axis is a count of the tachometeredges, both positive and negative. Data is only shown for the risingedges. For comparison, FIG. 14 illustrates the times between successivetachometer edges for a four pole (two phase, or two tachometer pulsesper revolution) brushless DC motor running at a steady speed.

[0056] A useful pattern that emerges from FIGS. 13 and 14 is that thenumber of phases is equal to one plus the number of successive instancesin which the time between tachometer pulses is less than the previoustime between pulses. Thus, by counting the number of successive timesthat the time between pulses decreases, the number of poles in the motorcan be determined.

[0057]FIG. 15 is a flow diagram illustrating an embodiment of a methodfor determining the number of poles in a brushless DC motor inaccordance with the present invention. Beginning at 200, a counter CNTRis initialized to zero. The time between a first tachometer pulse and asecond tachometer pulse is measured at 202 and assigned to the variableT1. The time between the second and a third tachometer pulse is measuredat 204 and assigned to the variable T2. At 206 and 208, CNTR isincremented if it is not zero. T2 is then compared to T1 at 210. If T1is not less than T2, the value of T2 is assigned to T1 at 212, and a newvalue of T2 (the next time between pulses) is determined at 204.

[0058] If T1 is less than T2 at 210, the counter CNTR is tested again at214. Here, if the counter has a nonzero value, it is the number ofphases in the motor, so the method stops at 216. Otherwise, the counteris again set to zero at 218, the value of T2 is assigned to T1 at 212,and a new value of T2 (the next time between pulses) is determined at204.

[0059] To increase reliability, the entire process illustrate in FIG. 15is preferably repeated a few times to confirm that the correct resulthas been obtained. The method shown in FIG. 15 can be used even when themotor is still starting up.

[0060] In some brushless DC motors, the time between successive pulseshas the opposite orientation. That is, the time between successivepulses keeps increasing before falling back down, rather than decreasingbefore rising back up. Therefore, the method illustrated in FIG. 15 ispreferably modified to also count the number of times T1 is successivelygreater than T2. Alternatively, a separate algorithm that counts thenumber of times T1 is greater than T2 can be employed when it has beendetermined that the motor being evaluated has the opposite orientation.

[0061] Another embodiment of a method for determining the number ofpoles in a brushless DC motor in accordance with the present inventionis to measure the time between different numbers of pulses, therebygenerating different sets of data, and then determining the data setwith the lowest amount of ripple. An example embodiment of this methodcan be illustrated with reference to FIGS. 16-19 which shows data takenfor a six pole (three phase) brushless DC motor. The data shown in FIG.16 is the amount of time between each successive tachometer pulse forthe motor. The data shown in FIG. 17 is the amount of time between everyother successive tachometer pulse. The data shown in FIGS. 18 and 19 isthe amount of time between every third and every fourth tachometerpulse, respectively. By comparing the relative amount of ripple in thedata, it is apparent that the motor has three phases because the datataken between every third pulse has the least amount of ripple.

[0062] As discussed above, and reiterated here, a tachometer edge orpulse refers not only to an edge or pulse in a position signal from anactual tachometer, but also more generally to anything that signifiesthe position of the rotor. Thus, the method described above fordetermining the number of poles in a motor can be implemented not onlywith an actual tachometer, but also with other methods for determiningrotor position such as the current sensing scheme described above withrespect to FIG. 8. The methods described herein for determining thenumber of poles are preferably implemented with a microprocessor ormicrocontroller located in, for example, control circuitry 24 in FIG. 8.

Synthesizing a Tachometer Signal

[0063] A problem associated with pulse width modulating the power supplysignal to a brushless DC motor is that the tachometer or other positionsensor within the motor typically relies on the motor power supply foroperation. Thus, the tachometer signal can be corrupted.

[0064] Another aspect of the present invention is a method forsynthesizing a tachometer signal. In one embodiment of such a method,the number of poles is determined, the period of one rotation isdetermined, and then the rotation period is divided by the number ofpoles to determine a synthesized tachometer period. This can beaccomplished by control circuitry that synchronizes the synthesizedtachometer signal using any suitable technique to initially determinethe rotor position. The synthesized tachometer signal can then be usedto synchronize pulses in the power supply signal to the rotor position.Preferably, the synthesized tachometer signal is periodicallyresynchronized using a position sensing scheme such as a tachometer orcurrent monitoring technique. The methods described herein forsynchronizing and/or synthesizing a tachometer signal are preferablyimplemented with a microprocessor or microcontroller located in, forexample, control circuitry 24 in FIG. 8.

[0065] Having described and illustrated the principles of the inventionin a preferred embodiment thereof, it should be apparent that theinvention can be modified in arrangement and detail without departingfrom such principles. Accordingly, such changes and modifications areconsidered to fall within the scope of the following claims.

1. A method for driving a brushless DC motor comprising: driving themotor with a pulse signal; and synchronizing the pulse signal to themotor position.
 2. A method according to claim 1 wherein synchronizingthe pulse signal comprises changing the frequency of the pulse signalresponsive to the speed of the motor.
 3. A method according to claim 1wherein synchronizing the pulse signal comprises initiating a pulse whenthe motor is at or near a minimum torque position.
 4. A method accordingto claim 1 wherein synchronizing the pulse signal comprises determiningwhen the motor is at or near a minimum torque position.
 5. A methodaccording to claim 4 wherein determining when the motor is at or near aminimum torque position comprises monitoring a tachometer coupled to themotor.
 6. A method according to claim 4 wherein determining when themotor is at or near a minimum torque position comprises monitoringcurrent from the motor.
 7. A method according to claim 6 whereinmonitoring current from the motor comprises detecting a commutationcurrent pulse.
 8. A method according to claim 4 wherein determining whenthe motor is at or near a minimum torque position comprises synthesizinga tachometer signal.
 9. A method according to claim 8 whereinsynthesizing a tachometer signal comprises: determining the number ofpoles in the motor; determining the length of time it takes the motor tomake one rotation; and dividing the length of time by the number ofpoles, thereby determining a synthesized tachometer signal period.
 10. Amethod according to claim 9 wherein synchronizing the pulse signalcomprises using the synthesized tachometer signal to determine when themotor is at or near a minimum torque position.
 11. A method according toclaim 1 wherein synchronizing the pulse signal comprises synchronizingpulses with less than all of the poles of the motor.
 12. A methodaccording to claim 1 further comprising driving the motor with multiplepulses per phase.
 13. A method according to claim 1 whereinsynchronizing the pulse signal to the motor position comprisesperiodically resynchronizing the pulse signal to the motor position. 14.A method according to claim 13 wherein resynchronizing the pulse signalcomprises: driving the motor with a shortened off time before the end ofa complete revolution; and driving the motor with an indeterminate ontime after the shortened off time.
 15. A method for determining thenumber of poles in a brushless DC motor, the method comprising:measuring a plurality of time periods from a position sensing devicecoupled to the motor; and analyzing the plurality of periods todetermine a pattern.
 16. A method according to claim 15 whereinanalyzing the plurality of periods comprises identifying a repetitivepattern in the periods.
 17. A method according to claim 15 whereinanalyzing the plurality of periods comprises: comparing each period tothe next successive period; and determining how many times a period isless than the next successive period.
 18. A method according to claim 15wherein analyzing the plurality of periods comprises: repetitivelymeasuring a first number of successive periods, thereby generating afirst group of data points having a first amount of ripple; repetitivelymeasuring a second number of successive periods, thereby generating asecond group of data points having a second amount of ripple; andcomparing the first and second amounts of ripple.
 19. A method accordingto claim 18 wherein analyzing the plurality of periods furthercomprises: repetitively measuring a third number of successive periods,thereby generating a third group of data points having a third amount ofripple; and comparing the third amount of ripple to the first and secondamounts of ripple.
 20. A method for starting a brushless DC motorcomprising: driving the motor with a constantly on power supply signal;monitoring the speed of the motor; and driving the motor with a pulsesignal when the motor has reached a suitable speed.
 21. A methodaccording to claim 20 wherein monitoring the speed of the motorcomprises counting a number of position events.
 22. A method accordingto claim 21 wherein the position events are tachometer pulses or edges.23. A method according to claim 20 wherein monitoring the speed of themotor comprises monitoring the actual speed of the motor.
 24. A drivecircuit for a brushless DC motor comprising: a switch constructed andarranged to drive the motor with a pulse signal responsive to a controlsignal; and control circuitry coupled to the switch and constructed andarranged to generate the control signal responsive to rotor positioninformation from the motor so as to synchronize the pulse signal to therotor position.
 25. A drive circuit according to claim 24 furthercomprising a current sensing device constructed and arranged to providethe rotor position information to the control circuitry by sensingcurrent flowing through the motor.
 26. A drive circuit for a brushlessDC motor comprising: means for driving the motor with a pulse signal;and means for synchronizing the pulse signal to the rotor position ofthe motor.
 27. A drive circuit according to claim 26 further comprisingmeans for sensing current flowing through the motor.
 28. A drive circuitfor a brushless DC motor comprising: a switch constructed and arrangedto drive the motor with a pulse signal responsive to a control signal; aposition sensing device coupled to the motor; and control circuitrycoupled to the switch and position sensing device, and constructed andarranged to measure a plurality of time periods from the positionsensing device and analyze the plurality of periods to determine apattern.
 29. A drive circuit for a brushless DC motor comprising: aswitch constructed and arranged to drive the motor with a pulse signalresponsive to a control signal; and control circuitry coupled to theswitch, and constructed and arranged to drive the motor with aconstantly on power supply signal, monitor the speed of the motor, anddrive the motor with a pulse signal when the motor has reached asuitable speed.